This invention relates to exhalation valves used where Patient End Expiratory Pressure (PEEP) is applied to an artificial respiratory circuit, and in particular to exhalation valves used where the applied Patient End Expiratory Pressure is adjustable.
When a patient is unable to breathe unaided, or requires assistance with breathing, the patient is usually connected to an artificial respiratory circuit including a ventilator programmed by a clinician to deliver an appropriate volume of air, or an air/oxygen mixture, to the patient. In such a respiratory circuit, it is desirable to prevent the patient from exhaling fully, and therefore the patient's lungs from deflating fully. This is because complete deflation, and subsequent reflation, of the patient's lungs requires a significant amount of the patient's energy.
Prevention of total exhalation is generally achieved by including a mechanism in the respiratory circuit which only allows exhaled breath above an appropriate exhalation pressure to escape the respiratory circuit through an exhalation port. Prevention of total exhalation in this way is known as applying “PEEP” to the respiratory circuit, where “PEEP” refers to Patient End Expiratory Pressure.
PEEP is currently applied to a respiratory circuit using either a so-called PEEP valve or an exhalation valve to control the passage of the exhaled breath through an exhalation port. A PEEP valve has a fixed and pre-determined release pressure for the exhalation port. An exhalation valve has a release pressure that is determined by the pressure of a gas within the hermetically sealed exhalation valve. This gas within the exhalation valve is usually supplied by the ventilator at a pressure that is determined and selected by a clinician.
Conventionally, exhalation valves comprise a chamber that is supplied by the ventilator, during use, with a gas under pressure, and a flexible membrane which defines a wall of the chamber and is disposed, in its relaxed state, slightly above the exhalation port of the respiratory circuit. In use, gas with a pressure selected by the user is supplied to the chamber by the ventilator. The supplied gas deforms the membrane elastically and outwardly from the chamber and into engagement with the exhalation port, thereby sealing the exhalation port. Therefore, in theory, the pressure of the exhaled gas within the respiratory circuit must exceed the pressure of the gas within the exhalation valve for the exhaled gas to be able to escape the respiratory circuit through the exhalation port. In this way, the pressure of the gas within the exhalation valve, which is determined and selected by a clinician, should equal the positive end expiratory pressure applied to the respiratory circuit by the exhalation valve.
Conventional membranes are usually formed in an elastomeric material, such as silicone rubber. There are a variety of different shapes of membrane currently in use, including a membrane having a generally top-hat shape where the central flat circular wall of the membrane is elastically deformable into engagement with the exhalation port, and a membrane having the form of a balloon where the membrane is elastically inflatable into engagement with the exhalation port.
However, a problem with exhalation valves of this type is that a certain amount of pressure, the “deforming pressure”, is required to elastically deform the membrane into engagement with the exhalation port. The PEEP applied to the respiratory circuit using conventional exhalation valves is not therefore the pressure of the gas within the exhalation valve. Instead, the PEEP applied to the respiratory circuit using conventional exhalation valves is equal to the pressure of the gas within the exhalation valve minus the deforming pressure. For example, a selected pressure of 10 cm H2O for the exhalation valve gas can deliver a PEEP as low as 8 cm H2O.
In order to address this problem, ventilators have been developed that include microprocessor controlled feedback for the pressure line to the exhalation valve so that this loss of pressure is automatically compensated for by increasing the pressure within the exhalation valve. However, such solutions are very expensive and require the ventilator, which is an expensive piece of equipment, to be replaced in order to solve the problem. For this reason, only a very small proportion of ventilators currently in everyday clinical use include a compensating pressure feedback mechanism, as discussed above.
There has now been devised an improved exhalation valve which overcomes or substantially mitigates the above-mentioned and/or other disadvantages associated with the prior art.